For the perforation of such workpieces by punching or drilling, at locations separated by selected distances from a reference surface such as the edge of a web or flange, it is known to transport the workpiece along a predetermined conveyor path and to sense the position of the reference surface for the purpose of properly disposing the perforating tool before actuating same.
Conventional sensing and positioning equipment used for this purpose is relatively complex and, in many instances, somewhat sluggish in its response to changing distances so as to limit the speed with which the workpiece can be moved through the apparatus. Typical prior-art devices of the type referred to utilize a single yieldable feeler (see, for example, U.S. Pat. No. 3,785,235), or a pair of such feelers (German open application No. 2,334,438) at least one of which must report contact with solid material to enable the operation of a perforator.